1. Field of the Invention
The present invention relates to a multilayer printed substrate employed for an information processing device, and a method of manufacturing an interlayer connection substrate at reduced cost.
2. Description of Related Arts
The recent trend of high operation rate of the interface such as the communication apparatus has resulted in an increase in both capacity and operation rate in the band for data transfer of the information processing devices, for example, routers, servers, RAID and the like, which has been doubled in the last three years. The transmission rate of the backplane at 10 Gbps has been put into practical application, and is expected to reach 25 Gbps for the next generation. It is therefore the pressing need to achieve high density of the server and cope with the high transmission rate of a midplane inside the rack. It is also highly demanded to realize the high density packaging of the module such as the blade connected to the midplane of the next mass production server with a press-fit connector.
FIG. 8 schematically illustrates the device structured to have mutual signal transmission via the midplane inside the server casing. For example, a server blade 105 is connected to the surface of the midplane, and a management module 106, a switch module, a fan module, a power supply module unit and the like are connected to the back surface of the midplane (FIG. 8 only shows the management module 106 as a representative example). The midplane 101 is a multilayer printed substrate including 24 layers, which is electrically coupled with the respective devices via a press-fit connector 103. The press-fit connector 103 is structured to achieve the electric coupling by inserting a spring-type press-fit connector pin 104 into a piercing through hole 102 (hereinafter referred to as a piercing TH) of the midplane 101. The generally employed structure has the single piercing TH occupied with the pin inserted from one side, and accordingly, the press-fit connectors are arranged to be positionally shifted mutually. However, as the number of types of the press-fit connector increases, and the density of the blade mounting is getting higher, arrangement flexibility has been demanded so as to allow arrangement of the press-fit connectors from both surfaces of the midplane.
In order to manufacture the substrate for both-surface press-fit connection through the generally employed process for forming the printed substrate such as sequential lamination for meeting the demand, the problem of increased process steps and higher costs is unavoidable. It is the urgent need to develop the technique for bonding both surfaces of the substrate, which allows electric coupling at an arbitrary position by bonding the multilayer substrates having the piercing THs. In other words, the adhesive layer having the conductive material such as solder buried is inserted among plural multilayer substrates for batch thermocompression bonding. This method allows adhesion and electric coupling between the multilayer substrates to be simultaneously conducted. The substrate manufactured through the aforementioned method will be referred to as the interlayer connection substrate.
Japanese Unexamined Patent Application Publication No. 2005-116811 discloses that the high-rate wiring board formed of the material selected to have the large cross-section area of the signal line, and the relatively low conductivity of the insulation layer, and the high-density wiring board formed of the material selected to have the relatively small cross-section area of the signal line, and the relatively high conductivity of the insulation layer are individually manufactured to cope with the high frequency in excess of 1 GHz, and high density component packaging at low cost. The thus manufactured wiring boards are bonded with the adhesive insulation sheet having through-vias preliminarily formed for interlayer connection so as to manufacture the multilayer wiring circuit substrate.
Japanese Unexamined Patent Application Publication No. 2001-326458 discloses that the individual supporting body for forming the wiring patterns are bonded with the prepreg (provided by preliminarily forming a via hole in the insulation substrate with adhesive layers on both surfaces, and filling the via hole with the conductive paste) to manufacture the multilayer wiring circuit substrate.
In order to apply the manufacturing processes as disclosed in Japanese Unexamined Patent Application Publication Nos. 2005-116811 and 2001-326458 to the midplane with mounted press-fit connector, it is necessary to overcome various problems, for example, preventing intrusion of the resin to the through hole (hereinafter referred to as TH) for the press-fit connector. It is therefore difficult to realize the application. Japanese Patent Application No. 2012-151288 discloses the technique as the solution for the aforementioned problem. The disclosed method of bonding the multilayer substrates uses the three-layered resin sheet having the adhesive layers pasted on both surfaces of the core layer with cured plastic material, and the Cu core solder plating ball. The present invention provides the technique for realizing the manufacturing of the interlayer connection substrate mounted with the press-fit connector at lower costs.